1. Field of the Invention
The present invention relates to magnetic recording and, particularly, to an improved system and method for interpolated timing recovery.
2. Description of the Related Art
In digital communications receivers, timing recovery circuits are used to acquire and then track the correct sampling and frequency of an analog signal. In an interpolated timing recovery circuit, this is accomplished by digitally resampling a stream of asynchronous samples of the analog signal. The digital resampling effectively reconstructs the values the signal takes on at points in time between the (asynchronous) times at which the signal was actually sampled. This is done by mathematically interpolating the asynchronous sample values of the signal. To allow for small errors in frequency, and to satisfy the Nyquist sampling criterion, the resampling period Ts is slightly longer than the asynchronous sampling period Ta.
In the case of magnetic recording, as shown in FIG. 1, data sectors 100 on magnetic disks are formatted to include an acquisition preamble 102, a sync or synchronization mark 104, and user data 106. Timing recovery uses the acquisition preamble 102 to acquire the correct sampling frequency and phase before reading the user data 106. The synchronization mark 104 demarcates the beginning of the user data. The preamble pattern is periodic, having period 4Ts, where Ts is the bit period.
The phase and frequency of the initial asynchronous samples of this waveform are unknown. The sampling phase (modulo the bit period Ts) can be anything, and the sampling frequency can be in error by as much as half a percent. Interpolated timing recovery includes an asynchronous phase to estimate the initial sampling phase and to initialize the interpolator appropriately using the estimate. Then, a synchronous acquisition step is used to refine the initial estimate of the phase and correct the sampling frequency.
This is illustrated more clearly with reference to FIG. 2. In particular, FIG. 2 illustrates various initial sampling phases of the 2T acquisition preamble. Points yk on the curve 1000 are the asynchronous samples, sampled at a sampling period of Ta. Points zk represent the interpolated (synchronous) points, at a resampling period of Ts. The values xcexck represent the fractions of the asynchronous period Ta at which to interpolate the next synchronous sample. The objective of zero phase restart (or asynchronous sampling) is to determine an initial interpolation interval xcexc0, i.e., the fraction of an asynchronous period Ta after the last asynchronous sample yk at which to interpolate the first synchronous sample.
One aspect of the present invention relates to an improved asynchronous sampling system and method, i.e., an improved zero phase restart system. Another aspect of the invention relates to synchronous acquisition, i.e., determining the phase error estimate. A timing error estimation unit is provided to calculate an acquisition timing error. The estimate is based on use of synchronous (interpolated) samples and an approximation to an arctangent function.
An interpolated timing recovery system according to the present invention is simpler to implement, requires fewer signal samples, and is more robust against signal distortions such as gain errors, DC offset errors, and magneto-resistive asymmetry.